Ink jet printing is a non-impact printing process in which a digital signal produces droplets of ink on media substrates such as paper or transparent films. It is widely used in industrial and office applications. Thermal or bubble jet drop-on-demand ink jet printers are the generally accepted medium price printers for personal computers. These printers have been widely received due to their high print quality, low cost, relatively quiet operation, environmental safety and graphics capability.
Both dyes and pigments have been used as colorants for ink jet printer inks. Dye colorants are generally disfavored because they are water-soluble even after drying, whereby they can be redissolved by contact with water and will run or smear when exposed to a water spill, or upon contact with felt pen markers and sweaty hands. In addition, they exhibit poor light stability and are known to fade even under conditions of office fluorescent lighting. These disadvantages of dye colorant ink jet inks prohibit their use in applications requiring water resistance and light stability.
Pigments are a preferred colorant in water-based ink jet inks because pigments are substantially water-insoluble and inherently more lightfast than dyes, thus providing improved water and smear resistance and improved lightfastness.
One of the major commercial objections to thermal ink jet printers is their relatively slow printing speed compared to printers using competing technologies, largely due to the rate of drying of the ink jet inks. In a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear.
Drying time can be lessened by including a cosolvent that is miscible with water and has a higher vapor pressure thereby accelerating drying by evaporation. This approach is disadvantageous because the solvent also evaporates more rapidly in the pen opening, which tends to result in obstruction or blockage of the openings (known as "crusting") in the pen nozzles after a period of non-use.
A preferred approach to improve the rate of drying is to use a cosolvent that causes rapid drying by increasing the rate of penetration of the ink into the media substrate. Such cosolvents, referred to hereafter as penetrants, although preferred, are not without their disadvantages. A particular problem with known penetrants is that they are not suitable for use with pigmented inks because they cause the pigment dispersions to flocculate and settle. In addition, the more commonly used penetrants have shown an incompatibility with the materials of construction of the pen body, in particular the swelling of organic polymeric materials.
Thus, there is a need for penetrants which decrease drying time, have improved crusting properties, which minimize the risk of material incompatibility and which are particularly suitable for use with the more preferred pigmented ink compositions.